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MPG calculations


sean f
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Just to settle an argument from work.

Does anyone know how the MPG calculations are made by the computers in most cars (including newer Land Rovers).

Mate recons that they just use the milage from the speedo and the fuel consumption from the level gauge in the tank.

I recon that the fuel gauge in the tank is just not accurate for this type of calc, especially when you get the instantaneous readouts. With modern electronics it should be possible for the computer to count the number of time an injector fires and since it is controlling the fuelling totalling up the number of times an injector fires and referencing it against a fuelling table should give a fuel consumption figure, then just use the speedo output (most are electric now) for distance.

Theoretically I can't see why this couldn't be done on a megasquirt setup if any one with sufficient knowledge was willing to take the time to write the code and add some hardware to read the distance / speed covered.

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Theoretically I can't see why this couldn't be done on a megasquirt setup if any one with sufficient knowledge was willing to take the time to write the code and add some hardware to read the distance / speed covered.

As a V8 owner do I need to be constantly depressed about consumption? :rolleyes::D

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Fuel flow isn't directly controled by throttle position, so measuring that alone I wouldn't have thought would be acurate enough for "real time" MPG. I imagine there is a fuel flow transducer measuring the exact fuel flow at a given time, if this is fed into the ecu along with the vehicle's speed it could output an acurate MPG figure.

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With modern engine management you can use a variety of sensors to get an idea of MPG. Some of the MegaSquirt crowd even managed realtime BHP & Torque gauges with a wideband O2 sensor :lol:

Fundamentally the big ones are injector duty cycle and RPM.

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Mate recons that they just use the milage from the speedo and the fuel consumption from the level gauge in the tank.

I recon that the fuel gauge in the tank is just not accurate for this type of calc, especially when you get the instantaneous readouts.

I agree with you, and post above. There are a variety of ways to work it out, but it definitely won't be done using the fuel tank gauge sensor, or the odometer reading!

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The fuel gauge sensor is usually only used for the "200miles remaining" type display.

The actual instantaneous MPG is calculated using an injector pulse width feed (or some analogous signal generated by the ECU) from the ECU to the dash (or the trip computer if its a seperate unit). It then takes this combined with current speed signal (generated by either a sensor on the gearbox, a sensor on the speedo head itself, or the ABS ECU) to give a mpg readout. As folk have said, on overrun with the RPM's above 1.5-2k most petrol engines wont inject any fuel whatsoever, so your MPG meter will read some large value or no value like "---"

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On an EFi or injected diesel it's pretty simple to work it out. You know the pressure on the fuel rail, the rating of the injectors and the time that each injector is open. From those you know exactly how much fuel is going into the engine in each cycle. All you need is a speed input and you have all he information you need.

For Megasquirt you can simulate the speed from the RPM and create a gauge in Megatune that shows instantaneous MPG if you are in a particular gear. You can do the same for an auto if it's locked up. Obviously that assumes you've correctly entered all the data about the injectors in the first place but you can always add a "fudge factor" if you find the gauge doesn't reflect the real MPG

If you look in the .ini files for Megatune on any of the more modern MS-Extra versions you'll see that such a gauge already exists, you just need to enter your gearing values into it.

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As a V8 owner do I need to be constantly depressed about consumption? :rolleyes::D

You'd be amazed how much fuel you can save driving on these guages. I can get my Dads Punto up to 72mpg on a 40mile round trip to town whereas if you ignore it and drive 'normally' it barely does 35mpg.

I think accurate fuel usage guages should be compulsory on new cars if everyone is really serious about saving fuel, some like the big colour changing glowing ball in a Honda Clarity maybe. I'd seriously consider fitting one to my 110 if I had the remotest idea how to.

Will :)

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